sysconf: Fix missing definition of UIO_MAXIOV on Linux [BZ #22321]

After commit 37f802f8 (Remove
__need_IOV_MAX and __need_FOPEN_MAX), UIO_MAXIOV is no longer supplied
(indirectly) through <bits/stdio_lim.h>, so sysdeps/posix/sysconf.c no
longer sees the definition.